One of the well known and long standing problems associated with heat pumps is that the outdoor coils thereof will, under normal circumstances, have frost accumulate thereon during the heating mode of operation. The overall efficiency of the heat pump system decreases significantly as the frost thickness increases; the decrease in efficiency results in valuable energy being wasted. Accordingly, many schemes have herefore been proposed for both detecting the frost and for taking corrective action so as to remove the frost from the outdoor coil. Examples of prior art systems include the following U.S. Pat. Nos.: 3,170,304; 3,170,305; and 3,400,553. Another prior art arrangement for solving the above stated problem is that disclosed in the co-pending application of Dale A. Mueller and Stephen L. Serber, Ser. No. 954,141, filed Oct. 24, 1978, now U.S. Pat. 4,209,994; this application discloses an arrangement for utilizing the temperatures of the outdoor air and the outdoor coil to modify the timing function for activation of the defrost mode of operation. In said co-pending application the primary temperature reference is the outdoor air temperature.
The present invention is an improvement over the arrangement disclosed in said co-pending application in that it dispenses with the outdoor air temperature sensor and relies only on the measurement of the outdoor coil temperature, using that temperature to approximate the temperature of the outdoor air for computation purposes of determining the timing function for initiating defrost. The present invention is a less costly system; it is an object of this invention therefore to provide a new, significantly improved and cost effective defrost control system for a reverse cycle refrigeration system.